Abstract: Through the exploration of the mechanism of welding cracks on beam hangers,the reason behind the crack is analyzed.Cracks are created by restrain stress on the steel which is embrittled due to the inducement and enhancement effects by hydrogen after quench hardening of the steel.Thus the contributing factors to welding cracks include quench hardening of steel,hydrogen and restrain stress.The cracks are proved by experiments to be cold welding cracks,and the cracks of casting weld junction are also proved to be a result of insufficient heat treatment of castings.A new process has been brought forward,using E4303 electrodes for backing weld and E5016 for second-layer welding,to eliminate cold crack resulting from insufficient heat treatment of the casting steel.